Boosted converter for pulsed electric machine control

ABSTRACT

A boost circuit is arranged to reduce rise and fall times of pulsed power used for pulsed control operation of electric machines. Magnetic energy present in the electric machine at the end of a pulse is extracted by the boost circuit to reduce the pulse fall time. The energy is stored by the boost circuit and then applied at the beginning of a subsequent pulse to reduce the rise time. By reducing rise and fall times compared to not using such a boost circuit, machine efficiency is improved.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No. 16/818,570, filed on Mar. 13, 2020, which claims priority of U.S. Provisional Application No. 62/819,097, filed on Mar. 15, 2019. U.S. application Ser. No. 16/818,570 is a Continuation-in-Part of U.S. application Ser. No. 16/353,166 filed Mar. 14, 2019 (now U.S. Pat. No. 10,742,155, issued on Aug. 11, 2020), which claims priority to U.S. Provisional Application Ser. No. 62/810,861 filed Feb. 26, 2019; U.S. Provisional Application Ser. No. 62/658,739 filed Apr. 17, 2018; and U.S. Provisional Application Ser. No. 62/644,912 filed Mar. 19, 2018. U.S. application Ser. No. 16/818,570 is further a Continuation of International Application No. PCT/US20/22262, filed Mar. 12, 2020. Each of the above listed provisional and non-provisional applications are incorporated by reference herein for all purposes.

BACKGROUND OF THE INVENTION

The present application relates generally to pulsed control of electric machines to selectively deliver a desired output in a more energy efficient manner, and more particularly, to a boosted converter circuit with improved rise and fall times for pulsing the electric machine.

The term “machine” as used herein is intended to be broadly construed to mean both electric motors and generators. Electric motors and generators are structurally very similar. Both include a stator having a number of poles and a rotor. When a machine is operating as a motor, it converts electrical energy into mechanical energy. When operating as a generator, the machine converts mechanical energy into electrical energy.

Electric machines can operate using either direct current (DC) or alternating current (AC).

Representative DC machines include brushless, electrically excited, permanent magnet, series wound, shunt, brushed, compound, and others.

With AC machines, there are two general varieties, asynchronous and synchronous. An example of an asynchronous electric machine is a three-phase induction motor.

Modern electric machines have relatively high energy conversion efficiencies. The energy conversion efficiency of most electric machines, however, can vary considerably based on their operational load. With many applications, a machine is required to operate under a wide variety of different operating load conditions. As a result, machines typically operate at or near the highest levels of efficiency at certain times, while at other times, they operate at lower efficiency levels.

Battery powered electric vehicles provide a good example of an electric machine operating at a wide range of efficiency levels. During a typical drive cycle, an electrical vehicle will accelerate, cruise, de-accelerate, brake, corner, etc. Within certain rotor speed and/or torque ranges, the electric machine operates at or near is most efficient operating point, i.e. its “sweet spot”. Outside these ranges, the operation of electric machine is less efficient. As driving conditions change, the machine transitions between high and low operating efficiency levels as the rotor speed and/or torque changes. If the electric machine could be made to operate a greater proportion of a drive cycle in high efficiency operating regions, the range of the vehicle for a given battery charge level would be increased. Since the limited range of battery powered electric vehicles is a major commercial impediment to their use, extending the operating range of the vehicle is highly advantageous.

A need therefore exists to operate electric machines, such as motors and generators, at higher levels of efficiency.

SUMMARY OF THE INVENTION

The present application is directed toward pulsed control of electric machines, such as motors and generators, to improve operational efficiency. In a non-exclusive embodiment, such a pulsed-controlled machine includes a power supply, a stator with windings, a rotor of design dependent upon the motor topology, a machine controller configured to selectively operate the machine in a pulsed mode, and a power converter coupled between the power supply and the electric machine. The power converter is arranged to provide pulsed input power to the windings of the stator of the machine in response to the machine controller. In addition, the power converter may include a boost circuit. The boost circuit is arranged to reduce rise and fall times of the pulsed power relative to the rise and fall times of the pulsed power without the boost circuit. The boost circuit extracts at least some of a magnetic energy present in the electric machine at the end of a pulse to reduce the pulse fall time, stores at least some of the energy, and applies at least some of the energy at the beginning of a subsequent pulse to reduce the rise time. By reducing the rise and fall times of the pulsed power, the efficiency of the electric machine and overall electrical system is improved.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and the advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a representative Torque/Speed/Efficiency graph illustrating the energy conversion efficiency of a representative electric motor under different operating conditions.

FIG. 2 is a graph illustrating a pulsed current signal applied to an electric motor.

FIG. 3A is a torque versus efficiency map for a motor operating at a fixed speed during a transition from zero to peak efficiency torque.

FIG. 3B is a torque versus work lost for an exemplary motor operating at a fixed speed during a transition from zero to peak efficiency torque.

FIG. 4 illustrates a pulsed controlled electric machine in accordance with a non-exclusive embodiment of the present invention.

FIG. 5A is a diagrammatic representation of a continuous three-phase AC waveform having a peak value of 50 Amperes.

FIGS. 5B and 5C are pulsed waveforms having a 50% duty cycle that provide the same power output as the continuous waveform of FIG. 5A.

FIG. 6A and FIG. 6B are representative circuits modeling the current flows through the three phases A, B and C of an exemplary motor.

FIG. 7A is a circuit diagram illustrating a prior art power converter.

FIG. 7B-7F are exemplary prior art timing diagrams showing switching states and voltages for the power converter shown in FIG. 5A.

FIG. 8 is a power converter including a boost circuit in accordance with a non-exclusive embodiment of the present invention.

FIGS. 9A-9C are signal diagrams illustrating how the boost circuit improves rise and fall times during pulse-controlled operation of the power converter in accordance with a non-exclusive embodiment of the invention.

FIG. 10 illustrates exemplary voltages available to drive an electric machine as a function of time in accordance with a non-exclusive embodiment of the invention.

FIG. 11 illustrates exemplary voltages available to drive an electric machine as a function of time in accordance with a non-exclusive embodiment of the invention.

FIG. 12 is another power converter including a boost circuit in accordance with another non-exclusive embodiment of the present invention.

FIG. 13 is a flow diagram illustrating steps for pulsed control operation of an electric machine in a vehicle in accordance with the present invention.

FIG. 14 is a diagram illustrating modulation of energy supplied to a motor in accordance with another embodiment of the invention.

In the drawings, like reference numerals are sometimes used to designate like structural elements. It should also be appreciated that the depictions in the figures are diagrammatic and not to scale.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present application relates generally to pulsed control of a wide variety of electric machines (e.g., electric motors and generators) that would otherwise be operated in a continuous manner By pulsed control, the machine is intelligently and intermittently pulsed on and off to both (1) meet operational demands while (2) improving overall efficiency. More specifically, under selected operating conditions, an electric machine is intermittently pulse-driven at more efficient energy conversion operating levels to deliver the desired average output more efficiently than would be attained by conventional continuous machine operation. Pulsed operation results in deliberate modulation of the electric machine torque; however, the modulation is managed in such a manner such that levels of noise or vibration are minimized for the intended application.

For the sake of brevity, the pulsed control of a wide variety of electric machines as provided herein is described in the context of a three-phase induction electric motor in a vehicle. This explanation, however, should not be construed as limiting in any regard. On the contrary, the pulse control as described herein can be used for many types of electric machine, meaning both electric motors and generators. For instance, the machine pulsed control as described herein may be used with any type of machine regardless if AC (e.g., induction, synchronous, any number of poles, etc.) or DC (e.g., brushless, electrically excited, permanent magnet, series wound, shunt brushed, compound, etc.). In addition, pulsed control of such electric machines may be used in any application, not just limited to electric vehicles. In particular, pulsed control may be used in systems that require lower acceleration and deceleration rates than vehicle applications, such as electric motors for heating, cooling, and ventilating systems.

Pulsed engine control is described in U.S. patent application Ser. No. 16/353,159 filed on Mar. 14, 2019, and U.S. Provisional Patent Application Nos. 62/644,912, filed on Mar. 19, 2018; 62/658,739, filed on Apr. 17, 2018; and 62/810,861 filed on Feb. 26, 2019. Each of the foregoing applications is incorporated herein by reference in their entirety.

Three-Phase Induction Machine

An induction machine includes two main components, a stationary stator and a rotating rotor. In a three-phase machine, the stator may include a three-coil winding that is excited by a three-phase AC input. When the three-phase AC input is passed through the three-phase winding, a rotating magnetic field (RMF) is generated. The rotational rate of the RMF is known as the synchronous speed (N_(s)) of the electric machine. The rotor is typically either a “squirrel cage” or a “wound” type rotor, both having a plurality of electrically conductive elements that are electrically shorted at their ends. In accordance with Faraday's law, the RMF induces a current within the conductive elements of the rotor. The induced current establishes an induced magnetic field, which interacts with the magnetic field produced in the stator coils. The interaction of the rotor and stator magnetic fields generates an electromagnetic force (EMF) causing the rotor rotation. This type of motor is called an induction motor because electrical current is induced on the rotor conductive elements by electromagnetic induction, as opposed to a direct electrically conductive path.

Three-phase induction motors provide a number of advantages. First, they are inherently self-starting. Second, the rotational speed of the rotor is easy to control. The rotational speed of the rotor (N_(r)) is always slightly less than the synchronous speed (N_(s)). This difference is known as slip, which may be expressed in terms of a percentage: Slip %=(N _(s) −N _(r))/N _(s)  Eq. (1)

The frequency of the three-phase AC power energizing the stator windings controls the RMF rotational rate and thus the synchronous frequency. In turn, the rotational speed of the rotor can be controlled based on Eq. (1) defined above.

While the frequency provided to the three-phase winding controls the synchronous speed (N_(s)), the amplitude of the applied AC controls the output torque of the electric machine. When the amplitude is higher or lower, the output of the machine is higher or lower, respectively.

Vehicle Motor Efficiency Map

Referring to FIG. 1 , an exemplary vehicle motor efficiency map 10 under different load and speed conditions is illustrated. The map 10 plots torque (N*m) along the vertical axis as a function of motor speed (RPM) along the horizontal axis. The maximum steady-state output power is given by curve 12.

The area under the peak-torque/speed curve 12 is mapped into a plurality of regions, each labeled by an operational efficiency percentage. For the particular motor shown, the following characteristics are evident:

-   -   The most efficient or “sweet-spot” region of its operating range         is the operating region labeled 14, which is generally in the         range of 4,500-6,000 RPM with a torque output in the range of         about 40-70 N*m. In region 14, the energy conversion efficiency         is on the order of 96%, making it the “sweet spot”, where the         motor is operating in its most efficient operating range.     -   As the motor speed increases beyond approximately 6,000+ RPM,         the efficiency tends to decrease, regardless of the output         torque.     -   As the output torque increases beyond 70 N*m or falls below 40         N*m, the efficiency percentage tends to decrease from its peak,         in some situations rather significantly. For example, when the         motor is operating at approximately 2,000 RPM and an output         torque of 100 N*m, the efficiency is approximately 86%. When         torque output falls below about 30 N*m, regardless of the motor         speed, the efficiency drops, approaching zero at zero load.     -   At any particular motor speed, there will be a corresponding         most efficient output torque, which is diagrammatically         illustrated by a maximum efficiency curve 16.

The map 10 as illustrated was derived from an electric motor used in a 2010 Toyota Prius. Map 10 is for an internal permanent magnet synchronous motor. It should be understood that this map 10 is merely illustrative and should not be construed as limiting in any regard. A similar map can be generated for just about any electric motor, for example a 3-phase induction motor, regardless if used in a vehicle or in some other application.

As can be seen from the map 10, the motor is generally most efficient when operating within the speed and torque ranges of the sweet spot 14. If the operating conditions can be controlled so that the motor operates a greater proportion of time at or near its sweet spot 14, the overall energy conversion efficiency of the motor can be significantly improved.

From a practical point of view, however, many driving situations dictate that the motor operate outside of the speed and torque ranges of the sweet spot 14. In electric vehicles it is common to have no transmission and as such have a fixed ratio of the electric motor rotation rate to the wheel rotation rate. In this case, the motor speed may vary between zero, when the vehicle is stopped, to a relatively high RPM when cruising at highway speeds. The torque requirements may also vary widely based on factors such as whether the vehicle is accelerating or decelerating, going uphill, going downhill, traveling on a level surface, braking, etc.

As can be seen in FIG. 1 , at any particular motor speed, there will be a corresponding most efficient output torque which is diagrammatically illustrated by maximum efficiency curve 16. From a conceptual standpoint, when the desired motor torque is below the most efficient output torque for the current motor speed, the overall efficiency of the motor can be improved by pulsing the motor, so as to operate the motor a proportion of time at or near its sweet spot and the remainder of the time at a low or zero torque output level. The average torque thus generated is controlled by controlling the duty cycle of sweet spot operation.

Referring to FIG. 2 , a graph 20 plotting total applied current to an electric motor on the vertical axis versus time on the horizontal axis is illustrated. The applied current may be the sum of the current on all three-phases for a three-phase motor. For illustrative purposes, it will be assumed that each ampere of applied current will produce 1 N*m of output torque. In this particular example, a desired motor output torque is 10 N*m, which will require 10 amps of current as represented by the dashed line 22. In this example, the most efficient torque output for the motor is 50 N*m corresponding to 50 amps of applied current.

During conventional operation, the motor would continuously generate 10 N*m so long as the desired torque remained at this value. With pulsed-control operation, the motor is pulsed, as represented by pulses 24, to deliver 50 N*m of torque for 20% of the time. The remaining 80% of the time, the motor is off. The net output of the motor therefore meets the operational demand of 10 N*m. Since the motor operates more efficiently when it is delivering 50 N*m than when it delivers 10 N*m, the motor's overall efficiency can thus be improved by pulsing the motor using a 20% duty cycle while still meeting the average torque demand.

In the above example, the duty cycle is not necessarily limited to 20%. As long as the desired motor output, does not exceed 50 N*m, the desired motor output can be met by changing the duty cycle. For instance, if the desired motor output changes to 20 N*m, the duty cycle of the motor operating at 50 N*m can be increased to 40%; if the desired motor output changes to 40 N*m, the duty cycle can be increase to 80%; if the desired motor output changes to 5 N*m, the duty cycle can be reduced to 10% and so on. Generally, pulsed motor control can potentially be used advantageously any time that the desired motor torque falls below the maximum efficiency curve 16 of FIG. 1 .

On the other hand, when the desired motor torque is at or above the maximum efficiency curve 16, the motor may be operated in a conventional (continuous or non-pulsed) manner to deliver the desired torque. Pulsed operation offers opportunity for efficiency gains when the motor is required to deliver an average torque below the average torque corresponding to its maximum operating efficiency point.

It should be noted that current and torque values and time scale provided in FIG. 2 are merely illustrative and are not intended to be limiting in any manner. In actual motor pulsing embodiments, the pulse duration used may widely vary based on the design needs of any particular system. In generally, however, the scale of the periods for each on/off cycle is expected to be on the order of 10 μsec to 0.10 seconds (i.e. pulsing at a frequency in the range of 10 to 100,000 Hz), as for example between 0.2 and 20 milliseconds (50 to 5000 Hz) as will be discussed in more detail below. Furthermore, there are a wide variety of different motors and each motor has its own unique efficiency characteristics. Further, at different motor speeds, a given motor will have a different efficiency curve. The nature of the curve may vary depending on the particular motor or a particular application. For example, the current pulses need not be flat topped as depicted in FIG. 2 and/or the current need not go to zero during the off periods but may be some non-zero value. Regardless of the particular curve used, however, at some proportion of the time the motor is operating is preferably at or near its highest efficiency region for a given motor speed.

Efficiency Improvements with Improved Rate of Torque Rise

The vast majority of current motor converters are typically designed for continuous, not pulsed operation. Such motors generally transition from the unenergized to an energized state relatively infrequently. As a result, little design effort is made in managing such transitions. To the extent any design effort is made in managing the transition, it is typically directed to achieving a smooth transition as opposed to a fast transition. The transition from the energized to energized states for most motors is therefore often rate limited (i.e., relatively not fast).

The Applicant has discovered that for a motor system that regularly transitions from an unenergized motor state to peak efficiency state such as with pulsed operation, even further efficiency improvements can be realized when the transitions occur as fast as possible. With fast transitions, for example from zero torque to the peak efficiency torque, the overall average motor efficiency is improved because the motor spends less time in transition where efficiency is less than the peak. This relationship is depicted in FIG. 3A and FIG. 3B

Referring to FIG. 3A, a torque versus efficiency map for an exemplary motor operating at a fixed speed (e.g. 6000 rpms) is illustrated. In the exemplary map, a range of torque outputs from 0.0 Nm to 250 Nm is plotted along the horizontal axis, while the efficiency of the motor from 0.0 percent to 100 percent is plotted along the vertical axis. The curve 26 depicts the transition of the motor from zero to peak efficiency torque. During this transition, as depicted by the shaded region 27, the peak efficiency torque has a much lower efficiency at the peak efficiency torque 28.

Referring to FIG. 3B, a map is provided illustrating torque versus work lost for an exemplary motor operating at a fixed speed during a transition from zero to peak efficiency torque. In this map, the work losses (W) are plotted along the vertical axis, while the torque output of the motor is plotted along the horizontal axis. As demonstrated by the curve 29, the work losses of the motor increase as the torque output increases during the transition from zero to peak efficiency torque. Therefore, the faster that transition time from zero to peak efficiency torque, the less work is performed and the less energy is consumed by the motor.

By substituting time in place of torque along the horizontal axis and then integrating the area under the curve 29, the energy consumed by the motor can be calculated for a given transition time. For instance, the Applicant found that with an exemplary motor, 7234.5 Joules of energy was used with a transition time of 0.5 seconds, while only 723.4 Joules of energy were used a transition time of 0.05 second. This comparison demonstrates that the faster the transition time from zero to peak efficiency torque, the lower the energy consumed in losses. It should be noted that with this example, it is assumed that no acceleration of the load has taken place, so no energy has been added to the load inertia.

For different motors, the transition of the motor from zero to peak efficiency torque, the peak efficiency torque and the work losses will all vary. The maps of FIG. 3A and FIG. 3B should therefore be viewed as merely exemplary and should not be construed as limiting in any regard.

Power Converter

Power inventers are known devices that are used with electric motors for converting a DC power supply, such as that produced by a battery or capacitor, into three-phase AC input power applied to motor stator windings. In response, the stator windings generate the RMF as described above.

Referring to FIG. 4 , a diagram of a power controller 30 for pulsed operation of an electric machine is illustrated. The power controller 30 includes a power converter 32, a DC power supply 34, an electric machine 36, and a pulse controller 38. The power converter 32 may be operated as a power inverter or power rectifier depending on the direction of energy flow through the system. When the electric machine is operated as a motor, the power converter 32 is responsible for generating three-phased AC power from the DC power supply 34 to drive the induction machine 36. The three-phased input power, denoted as phase A 37 a, phase B 37 b, and phase C 37 c, is applied to the windings of the stator of the electric machine 36 for generating the RMF as described above. The lines depicting the various phases, 37 a, 37 b, and 37 c are depicted with arrows on both ends indicating that current can flow both from the power converter 32 to the electric machine 36 when the machine is used as a motor and that current can flow from the electric machine 36 to the power converter 32 when the machine is used as a generator. When the electric machine is operating as a generator, the power converter 32 operates as a power rectifier and the AC power coming from the electric machine 36 is converted to DC power being stored in the DC power supply.

The pulse controller 38 is responsible for selectively pulsing the three-phased input power. During conventional (i.e., continuous) operation, the three-phased input power is continuous or not pulsed. On the other hand, during pulsed operation, the three-phased input power is pulsed. Pulsed operation may be implemented, in non-exclusive embodiments, using any of the approaches described herein, such as but not limited to the approaches described with regard to FIG. 5B, FIG. 5C and FIGS. 8 through 14 .

Referring to FIG. 5A-5C, plots are provided for illustrating the difference between continuous and pulsed three-phased input power provided to the induction motor 36. In each plot, current is plotted on the vertical axis and time is plotted along the horizontal axis.

FIG. 5A illustrates conventional sinusoidal three-phased input current 42 a, 42 b, and 42 c delivered to the induction machine 36. Phase B, denoted by curve 42 b lags phase A, denoted by 42 a by 120 degrees. Phase C, denoted by curve 42 c, lags phase B by 120 degrees. The sine wave period is T. The three-phased input current 42 a, 42 b, and 42 c is continuous (not pulsed) and has a designated maximum amplitude of approximately 50 amps. It should be appreciated that 50 amps is only a representative maximum current and the maximum current may have any value.

FIG. 5B and FIG. 5C illustrate two examples of different pulsed three-phased current waveforms 44 a, 44 b, and 44 c and 46 a, 46 b, and 46 c that each has a 50% duty cycle and peak amplitude of approximately 100 amps. As in FIG. 5A the period of the base sine wave is τ, however, now the sine wave is modulated on and off. The delivered current in FIG. 5B and FIG. 5C delivers the same average torque as the continuously applied three-phased input current of FIG. 5A (assuming torque is proportional to current, which is often the case). The difference between pulsed currents 44 a-c and 46 a-c is the duration of their respective current pulses and the interleaved “off” periods. In FIG. 5B, the current pulses 44 a-c are interleaved with “off” periods of equal length. The length of each on and off period is 2τ. In FIG. 5C, the current pulses 46 a-c and the interleaved “off” periods again have equal duration. In this case the duration is τ/2. In both examples, the duty cycle is 50%. However, the duration of the “on” and “off” time durations is different, i.e. the frequency of the pulsed modulation is different. The frequency of the pulsed modulation may vary based on the type of electrical machine used, noise and vibration considerations, current operating rotor speed, and other factors.

FIG. 5B and FIG. 5C illustrate applications in which the “on” motor drive pulses are evenly spaced while the motor is operated at a steady state desired output level. Such an approach works well in many circumstances but is not a requirement. The duty cycle need not be 50% but can be adjusted to match the desired average output torque. In FIG. 5B and FIG. 5C the phase of the on/off pulses is synchronized with the applied AC power; however, the phase of the on/off pulses need not be synchronized with the phase of the applied AC power in some embodiments. Thus, the relative sizes and/or timing of the motor drive pulses can be varied as long as they average out to deliver the desired average torque.

Motor Physics and Constraints

With any given motor, physics ultimately limits how fast a zero to peak efficiency torque transition can be. In general, the transition speed is based on the physics of how fast the electric fields can be built up in the motor, which in turn, are limited by the applied voltage, electric motor back emf (“BEMF”) and the inductance of the motor windings.

If we assume that the set point of the power converter 32 is incremented at time zero and the feedback is zero, then the control to the output stages of each phase will be saturated. As a result, either the low or high output power devices for each motor phase will be turned on hard. This results in six possible combinations, including:

-   -   1. Phase A and B positive with phase C negative,     -   2. Phase A positive with phase B and C negative,     -   3. Phase B and C positive with phase A negative,     -   4. Phase B positive with phase A and C negative,     -   5. Phase C and A positive with phase B negative, and     -   6. Phase C positive with phase A and B negative.

With each of these six possible combinations, the current flow in the motor 36 at time zero will be (a) the full current in one phase and (b) while the other two phases split the current. The ratio of these currents will depend, as further described below, by the rotor position at time zero.

Referring to FIG. 6A, a representative circuit modeling the current flows through the three phases A, B and C is shown.

Each phase A, B and C is represented by its self inductance (“LS”), it mutual inductance (“LM”), its resistance (“R”) and its BEMF.

In the case shown, Ic=Ia+Ib. The sum of the currents flowing in the mutual inductance is zero, and therefore, the mutual inductance has no effect on the current flow. The resulting reduced equivalent circuit assuming the BEMF of the motor is zero as illustrated in FIG. 6B. This circuit takes time to build the current to a given value:

$i = {\frac{V}{R}\left( {1 - e^{\frac{{- R}t}{L}}} \right)}$

If the BEMF is not zero, then the applied voltage to each phase will differ. Because the phase impedances and phase currents are balanced, the neutral point of the winding is for this case=Vbus*2/3. If winding B was connected to the negative rail then the neutral voltage would be=Vbus/3. This defines the currents Ia, Ib, and Ic for phases A, B and C as:

${Ia} = {\left\lbrack {{{Vbus} \times \frac{1}{3}} - {bemfa}} \right\rbrack \times \frac{\left( {1 - e^{\frac{{- R}t}{L}}} \right)}{R}}$ ${Ib} = {\left\lbrack {{{Vbus} \times \frac{1}{3}} - {bemfb}} \right\rbrack \times \frac{\left( {1 - e^{\frac{{- R}t}{L}}} \right)}{R}}$ ${Ic} = {\left\lbrack {{{Vbus} \times \frac{2}{3}} - {bemfc}} \right\rbrack \times \frac{\left( {1 - e^{\frac{{- R}t}{L}}} \right)}{R}}$

As all the values above are the instantaneous, the values at time zero are dependent upon the instantaneous value of the BEMF of each phase, which in turn is dependent upon the location of the rotor within one electrical cycle or pole pair pitch. It must also be noted that as time progresses, so does the instantaneous BEMF voltage per phase, the voltage applied to the motor inductance and the rate of rise of the motor phase current.

The intent is for the current to reach its desired value and phase to provide the demanded torque. The current is normally controlled using Field Oriented Control or “FOC”, and hence, the phase currents are transposed to the rotating frame values of “iq” (quadrature current) and “id” (direct current) where the vector sum of id and iq equals the peak magnitude of the phase current and Arc Tan id/iq is the angle. The cosine of the angle is the power factor. So deducing the values of id and iq using the Direct Quadrature Zero transform gives:

${id} = {{{Vb}{us}}*\left\lbrack {\frac{Cos\varnothing}{3} + \frac{\sin\;\varnothing}{\sqrt{3}}} \right\rbrack \times \frac{\left( {1 - e^{\frac{{- R}t}{L}}} \right)}{R}}$ ${iq} = {\left\{ {{{Vbus}*\left\lbrack {\frac{{Cos}\;\varnothing}{\sqrt{3}} - \frac{\sin\;\varnothing}{3}} \right\rbrack} + {Vpk}} \right\} \times \frac{\left( {1 - e^{\frac{{- R}t}{L}}} \right)}{R}}$

When inspecting the above formulae, the BEMF waveform Vpk only influences iq (quadrature current), both are influenced by the bus voltage, Vbus, and the angular position of the rotor, θ. Neither the angle nor the motor BEMF can be changed without a change of motor so the only parameter that can be controlled to affect the rate of rise of the phase currents, and hence the motor torque, is the applied bus voltage, Vbus. One aspect of this invention, therefore, proposes that the bus voltage be temporarily increased or “boosted” to a higher value than the normal operating bus voltage for the duration of transit time from zero to the peak efficiency torque during pulsing, thereby reducing that transit time.

It should be noted that when the converter is turned off the energy stored in the electric motor windings is returned to the bus voltage supply. If the supply cannot absorb this energy, then the bus voltage will rise as the bus capacitance absorbs this energy. Due to the amount of capacitance across the bus supply, this normal process will typically only increase the bus voltage by a small percentage, generally not enough to be considered as boosting the bus voltage. However, if this energy is captured independently, for example captured and stored in a storage device such as a capacitor or battery, then it could be recycled back to the motor in the form of a boost voltage.

Alternatively, during the “OFF” period the bus voltage could be augmented by a separate boost voltage source using a charge pump or separate voltage source. This boost supply should not be designed to charge the main bus capacitance but a separate capacitance that can be discharged into the motor over the on-transition time from zero to the demanded torque.

Conventional Power Converter Circuit

The inherent inductance of the motor can thus transitorily delay/slow the voltage/power steps between the on and off motor states. During continuous (non-pulsed) operation, these transitory effects tend to have a relatively minimal impact on overall motor operation. However, when rapid pulsing is used as contemplated herein, the transitory effects can have a larger net impact, and therefore, there is an incentive to reduce the leading and falling edge pulse transition times.

Referring to FIG. 7A, a circuit diagram of a representative prior art power converter 32 is illustrated. The power converter circuit 32 includes three pairs of switches, denoted as S1 thru S6. Each pair of switches S1-S2, S3-S4, and S5-S6 are connected in series between two voltage buses (+V_(BUS)) and (−V_(BUS)). The electrical potential between two voltage buses (+V_(BUS)) and (−V_(BUS)) is the available potential to operate the electric machine 36. Each of the switches, S1-S6, may have a bypass diode (D1-D6) connected electrically in parallel to the switch. These diodes help to prevent switch damaging voltage spikes that may be generated during switch operation.

The diodes also provide a path for recycling current which the switch may block. This is especially important when the electric machine 36 is used as a generator. The switches S1-S6 may be each be a MOSFET (metal-oxide semiconductor field-effect transistor) switch with integrated diodes. Alternatively, other types of transistors, such as, but not limited to, insulated gate bipolar transistors (IGBT) may be used.

A connection to a stator coil winding of the electric machine 36 is made between each switch pair. For phase A, the connection is between switch pair S1-S2 and is designated as 37 a. For phase B, the connection is between switch pair S3-S4 and is designated as 37 b. For phase C, the connection is between switch pair S5-S6 and is designated as 37 c.

Within the electric machine 36, each phase stator winding may be modeled as an inductor 31, a resistor 33, and a mutual inductance 35. These elements are only labeled in FIG. 7A for phase C, but analogous elements are present in the phase A and phase B windings.

The switches S1-S6 may be collectively referred to as a switching network that controls power to and from the electric machine 36.

When the electric machine 36 is operated as a motor, the switches S1-S6 operate in a conventional manner to apply current to each of the stator windings. For example, the switches may be operated as a six-step inverter, which provides AC power to the electric machine 36.

FIG. 7B shows the switching sequence to obtain a six-step output from the power converter 32. Each switch is open for ½ of a cycle period in a staggered manner. For each winding, current can flow through one switch on the top row and one or two switches on the bottom row. The switch pairs, S1-S2, S3-S4, and S5-S6 are never simultaneously turned on as this would short out the DC power supply 34.

FIG. 7C shows the voltage between points A and B as voltage V_(ab).

Similarly, FIG. 7D and FIG. 7E show the voltage between points B and C and C and A, respectively. Summing these voltages allows the voltage between each phase and neutral to be determined.

FIG. 7F shows the resultant phase voltage for phase A. The resultant 6-step waveform approximates a sine wave with a frequency, co, and is commonly referred to as the modulation signal. The phase voltage for phase B and C are shifted in phase by 120° and 240°, respectively, relative to the phase A voltage.

It should be appreciated that electric machine 36 may be operated as a generator as well as a motor. When operating as a generator, the energy flow is from the electric machine 36 to the DC supply 34. The power converter 32 acts as a 3-phase rectifier rather than an inverter.

In typical prior art systems, the switching network is used to control the power flowing to the electric motor by pulse width modulation (PWM) control. PWM control reduces the time that the switching network is in an active configuration of the switches S1-S6 where power can flow to the electric motor. That is, the fraction of time that the switches S1-S6 are in an inactive configuration, either S1, S3, and S5 or S2, S4, and S6 are all turned off, increases as the desired electric motor torque output decreases.

Power Converter with Boost

FIG. 8 shows a power converter circuit 132 including a boost circuit according to a non-exclusive embodiment of the current invention. As compared to the prior art power converter circuit 32 shown in FIG. 7A, power converter circuit 132 also includes additional switches SA and SB, which are each controlled by a pulse controller 38. They may be controlled by a common signal line 41 (as shown in FIG. 8 ) or they may have independent control lines (not shown in FIG. 8 ). When switch SA is turned on, the positive power supply voltage (+V_(DC)) is coupled to (+V_(BUS)). When switch SB is turned on, the negative power supply voltage (−V_(DC)) is coupled to (−V_(BUS)).

During operation, the pulse controller 38 operates to selectively turn switches SA or SB on and off by applying a pulsed waveform to signal line 41, which electrically connects pulsed controller 38 to switches SA and SB. When switch SA and SB are turned on, current may be delivered to the electric machine 36. Conversely, when SA and/or SB are turned off, no current, or only a transient current, is delivered to the electric machine 36.

The power converter circuit 132 also includes a capacitor C1, which has one conductive plate coupled to (+V_(BUS)) and the other conductive plate coupled to (−V_(BUS)). Collectively the switches SA and SB and the capacitor C1 may be referred to as a boost circuit, since their purpose is to increase the initial voltage on the +V_(BUS) and −V_(BUS) buses at the beginning of an “on” pulse as described below. In various embodiments, boost circuit may be incorporated into the switching network or may include elements distinct from the switching network.

As previously noted, the goal of pulsed motor control is to operate the electric machine 36 at substantially its most efficient level for the current machine speed during “on” periods and to cut-off power (provide zero or negligible power) during the “off” periods. For example, the power supplied during the off periods may be less than 10%, 5%, 1%, 0.5%, or 0.1% of the power supplied during the “on” period. The operating point while operating during the “on” period may have an efficiency within 5%, 2%, or 1% of a maximum operating efficiency point of the motor at the current motor speed. The transitions thru the low efficiency operating region between the “off” and “on” periods should be as fast as possible to maximize efficiency. Thus, the power transitions between the machine power “on” and “off” states ideally have a leading edge that transitions vertically straight up and a following edge that vertically transitions straight down. Such “perfect” pulses 60 are diagrammatically illustrated in FIG. 9A, which illustrates the ideal motor drive current versus time for pulsed control having a duty cycle of 50%. In this figure, the current pulse represents the sum of the current in all the phases. While the current pulse is shown as flat topped, this will not necessarily be the case.

In the real-world, a number of practical limitations make generation of such perfect pulses difficult to achieve. For instance, inductive aspects of both the electric machine 36 and the power converter 32 circuitry slow down the current rise and fall times. The actual response of a particular machine will vary with the electrical characteristics of the electric machine 36, the rotational speed of the electric machine and the available bus voltages. In general, the actual rise and fall of pulses occur more gradually, meaning the transitions occur over time. The nature of the rise and fall in the real-world is diagrammatically illustrated in FIG. 9B. As seen therein, there is a ramp-up period (rise time) 62 required for the current to actually rise from zero to the desired “on” power level and a ramp-down period (fall time) 64 required for the current to actually fall from the “on” power level down to zero.

During the power ramp-up and ramp-down periods, the electric machine 36 continues to consume or generate power. However, the machine operates less efficiently during these transition periods. In general, the machine efficiency will drop as the operating current drops from its maximum efficiency condition (curve 16 FIG. 1 ) towards zero, with the energy conversion efficiency getting noticeably worse as the current level approaches zero. Thus, the pulse distortion represented by the current ramp-up and ramp-down periods detract from efficiency gains resulting from pulsed operation. In general, the smaller the ratio of the rise/fall times to the pulse length, the less the transitory switching effects impact the machine's energy conversion efficiency during pulsing.

It should be appreciated that the transitory effects shown in FIG. 9B are illustrative in nature and do not necessarily reflect actual rise/fall times associated with operation of any particular electric machine. The relative scale of the rise time to the pulse length ratio can vary widely based on the characteristics of the machine used (which primarily dictates the rise and fall times), the frequency of the pulsing (which is primarily dictated by the control scheme used) and the pulse width (which is dictated by the control scheme and machine load). The voltage available to power the electric machine and machine rotation speed will also impact the pulse rise and fall times. If the pulsing is slow compared to the machine response, the rise/fall times may be a small fraction of the pulse width and the transitory switching effects may have a minimal impact on machine performance Conversely, if the pulsing is very rapid and/or the machine response is low, the rise/fall times may be a significant fraction of the pulse width and can even exceed the pulse width in some situations. If not managed carefully, the transitory efficiency losses associated with switching can significantly reduce or even eliminate any theoretical gains that can be attained by pulsed operation. Thus, it is important to consider the transitory switching effects associated with pulsed operation when determining the pulsing frequency and control schemes that are appropriate for any particular application.

The capacitor C1 included in the power converter circuit 132 of FIG. 8 is provided to improve the current rise and fall times. The capacitor C1 may store energy from the electric machine 36 during the ramp-down period and supply energy to the electric machine 36 during the ramp-up period. This results in faster turn-on and turn-off transitions than would occur without the capacitor C1.

To better understand operation of the power converter 132, assume the power converter 132 is initially in an “on” state and the electric machine 36 is operating as a motor. This implies that, the switches SA and SB are turned on so that current can flow from the positive terminal of the DC supply 34 thru the power converter 132 to the electric machine 36 and return to the negative terminal of the DC supply 34. The switches S1 thru S6 will oscillate in the configurations shown in FIG. 7B to apply AC power to the electric machine 36.

To terminate motor operation the switches SA and SB may be turned off, allowing the +V_(BUS) and −V_(BUS) buses to have a different potential than their respective terminals of the DC power source 34. Since the circuit is now open, current must cease to flow thru the circuit; however, there may be significant energy associated with current generated magnetic fields in the electric machine 36. At least some of this energy may be extracted from the electric machine 36 and is captured in capacitor C1 where it is stored. This will increase the electrical potential difference between the positive voltage bus and negative voltage bus. For example, the potential on line +V_(BUS) may increase above that of the positive terminal of the DC power source, +V_(DC), and the potential on line −V_(BUS) may decrease below that of the negative terminal of the DC power source, −V_(DC). Note that the switches S1-S6 all have bypass diodes, which allow unidirectional current to flow from the electric machine 36 to the +V_(BUS) line and from the −V_(BUS) line to the electric machine 36 independent of the switch position. Coincident with, or nearly coincident with, the turning off of switches SA and SB any of the switches S1-S6 that may have been turned on when switches SA and SB opened are turned off, so that current does not flow through any of these switches between the +V_(BUS) and −V_(BUS) lines to the electric machine 36.

When motor operation is once again desired, the switches S1-S6 may be turned on in one of the patterns shown in FIG. 7B. The switching pattern must correspond to the rotor rotation angle so that the phase of the applied current matches the correct phasing to once again supply power to the electric machine 36. Switches SA and SB are closed when the voltage on the +V_(BUS) drops to +V_(DC) and −V_(BUS) rises to −V_(DC), respectively. This circuit configuration and control method is arranged to provide a higher initial voltage to be applied to the electric machine 36 at the start of the “on” phase, which advantageously reduces the pulse rise time.

FIG. 10 illustrates exemplary +V_(BUS) and −V_(BUS) waveforms versus time for the circuit shown in FIG. 8 . The pulse generator 38 generates a digital waveform 43 consisting of a string of digital “0's” and “1's”. The 1's may correspond to the electric machine 36 being turned “on” and the 0's may correspond to the electric machine being turned “off”. In FIG. 10 , the electric machine 36 is being pulsed at a 40% duty cycle; however, this is exemplary only and any duty cycle may be used. The voltage 45 on the +V_(BUS) rail increases to +V_(BOOST) during the ramp-down period and the voltage 47 on the −V_(BUS) rail drops to −V_(BOOST) during the ramp-down period. The magnitude of the voltage change on the +V_(BUS) and −V_(BUS) buses may be equal or different. The +V_(BUS) voltage 45 and −V_(BUS) voltage 47 remain relatively constant during the motor off time as the energy from the motor ramp-down is stored in capacitor C1. When the pulse generator waveform 43 returns to a digital “1”, the energy stored in capacitor C1 is supplied to the electric machine 36 thru the switch array S1-S6. This causes the voltage 45 on the +V_(BUS) rail to return to +V_(DC) and the voltage 47 on the −V_(BUS) rail to return to −V_(DC) as the charge in capacitor C1 is dissipated and the energy stored in capacitor C1 is used to drive the motor. Motor operation during the “on” period is sustained by turning on switches SA and SB, so that energy from the DC power supply 34 may be used to drive the motor. Effectively the boost circuit increases an available electric potential between the positive voltage bus and the negative voltage bus to drive the electric machine at the beginning of at least one pulse in the series of pulses. It should be appreciated that positive voltage bus and negative voltage bus are relative terms and the electric potential on each of these buses relative to ground potential may be either positive or negative. The boost circuit may be used to increase the available electric potential to drive the electric machine for all pulses in the series of pulses.

While the exemplary power converter with boost circuit is shown in FIG. 8 as having a switch adjacent both the positive and negative terminal of the DC power supply, this is not a requirement. In some embodiments, only a single switch may be required

The switches SA and SB in conjunction with the capacitor C1 can thus be used to reduce the power rise and fall times, in some cases by factors of 2, 5, 10 or more. The voltage across capacitor C1 can be increased above that of the power supply by storing energy recovered from the motor during its ramp down. The magnitude of the voltage increases with the amount of magnetic energy that can be extracted and captured. This can significantly reduce potential deleterious transitory switching effects associated with pulsed operation.

Examples of improved rise and fall times are schematically shown in FIG. 9C. As evident in the figure, the ramp-up rise time 66 on the pulse leading edge is faster/shorter as compared to the corresponding ramp-up time 62 shown in FIG. 9B. Similarly, the ramp-down time 68 of the pulse trailing edge is faster/shorter as compared to the corresponding ramp-down time 64 shown in FIG. 9B. Therefore, it should be appreciated that electric machines designed with pulsed control in mind or modified to improve the transient response of the machine to power pulses, can benefit even more from pulsed operation than existing machines.

It should be appreciated that the appropriate pulsing frequency implemented by the pulse controller 38 for different machines may be very different based on the machine's construction, operating environment and operational range. For some electric machines, switching frequencies on the order of 10-50 kHz may be appropriate—whereas for other machines much lower switching frequencies, as for example 10-500 Hz range may be more appropriate. The most appropriate pulsing frequency for any particular machine will depend on a wide variety of circumstances, such as the type of machine, the load of the machine, and/or the application of the machine.

It should be appreciated that the details of the boost circuit used to shorten the rise and fall times of the power to or from an electric machine may vary depending on the type of electric machine and its operating regime. For example, in some cases the one of the switches SA or SB can be deleted from the power converter circuit 132. Other types of power converter circuits and control strategies may be used. For example, a Z-source inverter, where a diode, two inductors, and two capacitors are situated between the power supply and switching network may be used in some situations.

The voltage boost level and size of capacitor C1 can be chosen appropriately for the electric machine and its inductive and resistive characteristics to shorten the transient rise/fall times associated with pulsing the machine on and off. Preferably, the respective capacitance and boost voltage levels are also selected to maximize overall machine efficiency during pulsing, including inefficiencies associated with the transients themselves and the effects of any overshoot that may occur due to use of the capacitor C1. Since the capacitor C1 is used to improve transient response, it may be opportunistically recharged in the periods when the motor is not being supplied power—as for example during the electric machine off periods. This mode of operation is explained in more detail in the description below regarding FIG. 11 .

Depending on the motor speed and load there may be insufficient energy stored in the magnetic fields of the motor to adequately boost the +V_(BUS) and −V_(BUS) voltages for sharp rise and fall times. In such cases it may be desirable to boost the potential difference across the electric machine during the off periods between pulses. An exemplary voltage waveform showing two boost cycles 73 a and 73 b is shown in FIG. 11 . It should be appreciated that more or less than two boost cycles may be used depending on the operating conditions of the electric machine. An appropriate switching network and control strategy is required to implement this type of control.

Referring to FIG. 12 , another power converter 200 including a boost circuit 202 in accordance with another embodiment of the invention is illustrated. The power converter 200 includes switches S1 and S2 for phase A, switches S3 and S4 for phase B and switches S5 and S6 for phase C. Each pair of switches S1-S2, S3-S4, and S5-S6 are connected in series between two voltage buses (+V_(BUS)) and (−V_(BUS)). The electrical potential between two voltage buses (+V_(BUS)) and (−V_(BUS)) is the available potential to operate the electric machine 36. The switches S1-S6 are collectively referred to as a switching network that controls the power to and from the machine 36. When operating as a motor, power from a DC supply is provided via the switching network of switches S1-S6. In turn, the switching network provides phased energy to the three phases of stator windings of the machine 36 as previously described. Similarly, when operating as a generator, the energy flow is from the machine 36 to a storage device, such as a battery.

The boost circuit 202 includes a boost supply 204, switch 206, a capacitor C1, a battery and a control signal 208 generated by the pulse controller 38. As the pulse controller 38 was previously described, a detailed explanation is not repeated herein for the sake of brevity.

In various embodiments, the boost supply 204 can be a dedicated circuit (e.g., charge pump or separate voltage source) capable of generating a boost voltage and/or a storage device such as another capacitor and/or battery. With the later embodiments, at least some of the energy stored by the storage device may be derived from the motor 38 itself. For example, when the machine 36 is operating as a generator, or when the machine 36 is acting as a motor and transitions from on to off states, such as during pulsing, the produced energy can be diverted to and saved certain components in the boost circuit 202, such the capacitor C1 and/or the battery. The saved energy can then be used to “boost” the positive rail (+V_(BUS)) during positive transitions as described below.

The switch 206 can be any type of switch that is capable of switching between the positive (+) and negative (−) electrodes of the boost supply 204. It is anticipated that this switch will be constructed using semiconductor devices. In a specific but non-exclusive embodiment, the switch 206 is a single pull double throw switch.

During continuous motor operation, phased power is provided to the stator windings of the machine 36 via the switches S1 and S2 for phase A, switches S3 and S4 for phase B and switches S5 and S6 for phase C as is well known in the art. The net result is a continuous torque output of the motor as previously described.

During pulsed operation, the pulsed controller 38 controls the switch 206 via the control signal 208 to control the boost circuit 202. With a positive pulse transition, the switch 206 is activated to connect the positive rail (+V_(BUS)) to the positive (+) terminal of the boost supply 204. As a result, the boost supply 204, operating in cooperation with the capacitor C1 and battery, act to boost the voltage on the positive rail (+V_(BUS)). With the Increased or Boosted Voltage on the Positive Rail, the transition time is reduced. Once the stored energy in the boost circuit has diminished or the peak torque level has been achieved the control signal 208 directs the switch to connect the positive rail (+V_(BUS)) to the negative (−) terminal of the boost supply 204. As a result, the boost voltage is effectively removed from the positive rail (+V_(BUS)).

The effect of the boost circuit 202 is also illustrated in FIGS. 9A-9C. In particular, FIG. 9A shows an ideal pulse with no transition time, FIG. 9B shows a “real world” pulse with a transition time designated by reference numeral 62. As previously noted, inductive aspects of both the electric machine 36 and the circuitry of the power converter 200 slow down the current rise and fall times. FIG. 9C shows a transition aided by the boost circuit 202. As can be readily understood by a comparison, the “boosted” transition time 66 as shown in FIG. 9C is significantly smaller (i.e. faster) than the transition time 62 as shown in FIG. 9B.

With the FIG. 12 embodiment, the capacitor C1 is arranged in parallel with each of the switch pairs S1-S2, S3-S4 and S5-S6 between the positive rail (+V_(BUS)) and negative rail (−V_(BUS)). In a non-exclusive embodiment, the size of C1 is derived from the ripple current of the power converter 200 when acting as an inverter. With this arrangement, the ability of the boost circuit 202 to reduce the rise and fall times of pulses is improved.

Operational Flow Diagrams

FIG. 13 is a flow diagram 70 illustrating steps for pulsed control operation of an electric motor with characteristics such as those depicted in FIG. 1 .

In the initial step 72, the current motor output and current motor speed are ascertained.

In decision step 74, a determination is made based on the current motor output and current motor speed if the motor should be operated in a continuous mode or a pulsed mode. In other words, a determination is made if the desired motor torque is above or below the most efficient output torque for the current motor speed (i.e., the maximum efficiency curve 16 of the motor map illustrated in FIG. 1 ). If above, the motor is operated in the continuous mode. If below, the motor may advantageously be operated in the pulsed mode.

In step 76, the motor is operated in the continuous mode 76 if the current motor torque is above the most efficient output torque for the current motor speed.

In step 78, the power output or magnitude of the “on” pulses that provide for substantially maximum efficiency operation at the current motor speed is determined.

In step 80, the desired pulse duty cycle for operation in the pulsed mode is determined so that the average output power or torque matches the desired output.

In step 82, the motor is operated in the pulsed mode using the determined pulse duty cycle and pulsed power output. The use of the power controller 30 with the boosted power converter circuits 132 or some other power converter circuit capable of storing and releasing magnet energy from the electric machine reduces, often significantly, the rise and fall times of the pulses, further improving motor efficiency.

The above steps 72-82 are continuously performed while the motor is in operation. At any particular motor speed, there will be a corresponding most efficient output torque which is diagrammatically illustrated by maximum efficiency curve 16 in FIG. 1 . As the instantaneous motor output request and/or current motor speed change, a decision is made to operate the motor in either the continuous or pulsed mode as appropriate. From a conceptual standpoint, when the desired motor torque is below the most efficient output torque for the current motor speed, the overall efficiency of the motor can be improved by pulsing the motor. As a result, for electric motor-powered vehicles the overall efficiency of the vehicle is improved, meaning the vehicle range between battery recharging is extended.

FIG. 14 is a diagram illustrating a system 300 for modulating the energy supplied to a machine 36 in accordance with another non-exclusive embodiment of the invention. The system 300 includes the machine 36, a power converter 32, a torque control decision module 302, a feedback sensor 304 for generating a feedback signal 306 indicative of the angular position of the rotor of the machine 36 and a torque and speed estimator 308.

During operation of the system 300, the torque modulation decision module 302 receives a torque demand. In response, the torque modulation decision module 302 makes a determination if the requested torque is less than the peak efficiency torque of the machine 36 when operating as a motor.

If not, meaning the torque demand is larger than the peak efficiency torque, the machine 36 is operated as a motor in the continuous mode. In which case, the torque demand waveform 310 provided to the power converter 32 is indicative of continuous operation of the machine 36 operating as a motor.

On the other hand if the torque demand is less than the peak efficiency torque of the machine 36, then the machine 36 is operated as a motor in the pulsed mode. In which case, the torque modulation decision module 302 produces a modulated waveform 310 for the power converter 32, causing the machine 36 operating as a motor to switch or pulse between the peak efficiency torque of the motor and a lower torque, the average of which is substantially equal to the demanded torque. In various embodiments, the lower torque can be zero, but it is not necessarily zero. The lower torque can be some other torque value above zero, provided the average of the lower and peak efficiency torque is substantially equal to the demanded torque.

The power converter 32 includes a switching network including pairs of switches S1-S2 for phase A, switches S3-S4 for phase B and switches S5-S6 for phase C, all of which are not shown in the figure for the sake of simplicity. As previously noted, the switches S1-S6 are controlled by the power converter 32 to operate the machine 36 either (1) continuously as a motor resulting in generating a continuous torque output when the torque demand is greater than the peak efficiency torque or (2) in the pulse mode when the torque demand is less than the peak efficiency torque. The power converter 32 can control the energy supplied to the machine 36 using any of a number of different protocols, such as Pulse Width Modulation (PWM), Direct Torque Control (DTC), hysteresis, or “dead beat” control, which is a form of current modulation.

In alternative embodiments, a boosted power converter such as 132 of FIG. 8 or 200 of FIG. 12 may be used. With boosted versions of the power converter 32, efficiency and performance of motor operation of the machine 36 is improved due to the faster rise and/or fall times of pulses during pulsed operation.

The feedback sensor 304 generates the feedback signal 306, which is indicative of the angular position of the rotor of the machine 36. The feedback signal is provided to each of the power converter 32 and the torque and speed estimator 308. With the angular position of the rotor known, the torque and speed estimator 308 can provide accurate estimates of the torque and speed of the motor to the torque modulation decision module 302. In response, the waveform 310 can be adjusted as necessary so that the timing of switching network (i.e., the timing of turning the switches S1-S6 on/off) within the power converter 32 can be precisely controlled so that each of the phases A, B and C of energy are timed to coincide with the current position of the rotor. As a result, the operation of the machine 36 as a motor is both smooth and efficient. It should be noted that the use of a feedback sensor 304 is not mandatory and that other techniques can be used for measuring or estimating the angular position of the rotor of the machine 36. For instance, any of a number of sensorless approaches may be used as well.

Other Motor and Generator Types

There are a wide variety of machines, both electric motor and generator, that are known and commercially available, including both DC and AC motors/generators. Although the structure, control and energy conversion efficiency of the various types of electric motors and generators vary significantly, most electric motors and generators are designed to operate over a range of operating conditions and their energy conversion efficiency will vary over that operating range, often significantly. In general, the control principles described herein can be applied to any type of machine to improve efficiency, provided the operating range includes regions below the equivalent of the maximum efficiency curve 16 illustrated in FIG. 1 .

Some prior art motors are currently operated using pulse width modulation (PWM) control. However, such motors are driven without consideration of what might be their most efficient energy conversion level. As such, the described approach can also be used to improve the energy conversion efficiency of such motors as well.

Many types of motors, including brushless DC motors, induction motors, synchronous AC motors, switched reluctance motors, etc. are traditionally driven by a continuous, albeit potentially varying, drive current to deliver the desired torque output. Frequently, the drive current is controlled by controlling the output voltage of an inverter and/or converter (which serves as the voltage input to the motor). Generally, by changing the relative phasing between the rotor and stator magnetic fields a motor can be operated as a generator. Thus, circuits and control methods described in terms of a motor are equally applicable to using an electric machine as a generator. The described pulsed control is particularly beneficial when such motors and generators are operated in regions below their respective maximum energy conversion efficiency points.

Therefore, the present embodiments should be considered illustrative and not restrictive and the invention is not to be limited to the details given herein but may be modified within the scope and equivalents of the appended claims. 

What is claimed is:
 1. An electric machine, comprising: a stator; a rotor; a winding; a storage device; and a controller configured to operate the electric machine in a pulsed mode such that: during on pulses, the electric machine is in an on state and generates a work output; and between the on pulses the electric machine is in an off state and generates substantially no work output, the winding and the storage device further cooperating while the controller is operating the electric machine in the pulsed mode such that: during first transitions from the on state to the off state at the end of on pulses, energy is harvested and stored in the storage device; and during second transitions from the off state to the on state at the start of next on pulses, the harvested energy stored in the storage device is applied to the electric machine, wherein the applied harvested energy acts to reduce transition times of the second transitions relative to if the harvested energy is not applied respectively.
 2. The electric machine of claim 1, wherein the energy that is harvested from the winding is magnetic energy from the winding.
 3. The electric machine of claim 1, wherein the storage device for storing the harvested energy is coupled between a first power rail and a second power rail, wherein the harvested energy stored by the storage device is applied so that a potential difference between the first rail and the second rail is increased during the second transitions.
 4. The electric machine of claim 3, wherein the storage device is a capacitor having a first plate coupled to the first power rail and a second plate coupled to the second power rail.
 5. The electrical machine of claim 4, wherein the capacitor is configured (a) to store the energy harvested during the first transitions and (b) apply the stored energy so as to increase the potential difference between the first power rail and the second power rail during the second transitions respectively.
 6. The electric machine of claim 1, wherein the controller is further configured to selectively control operation of the electric machine in either a continuous mode or the pulsed mode.
 7. The electric machine of claim 6, wherein the controller is further configured to: (a) ascertain if a requested output for the electric machine is more than or less than a threshold for the electric machine; (b) operate the electric machine in the continuous mode if the requested output is more than the threshold; and (c) operate the electric machine in the pulsed mode if the requested output is less than the threshold.
 8. The electric machine of claim 1, wherein the controller is further configured, while operating the electric machine in the pulsed mode, to: (d) define an amplitude for one or more on pulses, the defined amplitude at or above a threshold; (e) define a duty cycle for the one or more on pulses; and (f) define a frequency for the one or more on pulses, wherein the amplitude, duty cycle and the frequency are defined so that the work output of the electric machine during the one or more on pulses matches a requested output of the electric machine.
 9. The electric machine of claim 1, wherein the first transitions occur on falling edges of the on pulses and the second transitions occur on the leading edges of the on pulses respectively.
 10. The electric machine of claim 1, further comprising a power converter coupled between a power supply and the electric machine, the power converter arranged, during operation in the pulsed mode, to either: deliver pulsed power to windings of the electric machine while operating as a motor; or receive pulsed power from the windings of the electric machine when operating as a generator.
 11. The electric machine of claim 10, further comprising a boost circuit including the storage device and one or more switches coupled between a power supply and a switching network that controls power to and/or from the electric machine.
 12. The electric machine of claim 11, wherein the boost circuit further includes a boost power supply that is selectively coupled to either or both of a first power rail and/or a second power rail by the one or more switches.
 13. The electric machine of claim 12, wherein the boost circuit effectively increases a voltage potential between the first power rail and the second power rail at the leading edge of the second transitions respectively.
 14. The electric machine of claim 1, further comprising a switching network including a pair of switches coupled between the first power rail and the second power rail for each phase of the electric machine respectively, the harvested energy sequentially applied for each phase of the switching network when the electric machine is operating in the pulsed mode.
 15. The electric machine of claim 1, wherein the electric machine is one of the following: a DC motor or generator; an AC motor or generator; an induction machine; a pulse width controlled electric machine; or a synchronous AC machine.
 16. An apparatus, comprising: a power converter coupled between a power supply and an electric machine, the power converter arranged to operate the electric machine in a pulsed mode by alternatively transitioning the electric machine between: (a) pulsed on periods during which the electric machine operates above a predefined efficiency threshold and generates a work output; and (b) second periods between the pulsed on periods in which the electric machine generates substantially no work output; and a boost circuit coupled to the power converter, the boost circuit causing a reduction of transition times of leading edges of the pulsed on periods relative to transition times without the boost circuit.
 17. The apparatus of claim 16, further comprising a decision module configured to receive a demand signal, and in response thereto, make a decision to operate the electric machine in either a continuous mode or in the pulsed mode in which the electric machine transitions between (a) the pulsed on periods and (b) the second periods between the pulsed on periods.
 18. The apparatus of claim 17, wherein the decision module operates the electric machine in the continuous mode when the demand signal is above the predefined efficiency threshold and in the pulsed mode when the signal is below the predefined efficiency threshold.
 19. The apparatus of claim 16, further comprising a decision module configured to receive a demand signal, and in response thereto: make a decision to operate the electric machine in the pulsed mode if the received demand signal is below the predefined efficiency threshold; and generate a modulated waveform for operating the power converter in the pulsed mode so that the electric machine transitions between (a) the pulsed on periods and (b) the second periods between the pulsed on periods, wherein an average output of the electric machine during the pulsed periods and the second periods meets the demand signal.
 20. The apparatus of claim 19, further comprising a feedback sensor for generating a feedback signal indicative of an angular position of a rotor of the electric machine, the decision module further configured to use the feedback signal to adjust the modulated waveform.
 21. The apparatus of claim 20, further comprising a torque and speed estimator configured to provide to the decision module a torque and a speed estimation of the electric machine in response to the feedback signal.
 22. The apparatus of claim 16, wherein the power converter includes a switching network, the switching network including a pair of switches coupled between a first power rail and a second power rail for each phase of the electric machine respectively.
 23. The apparatus of claim 16, wherein the boost circuit is further configured to harvest and store energy from a winding of the electric machine during falling edges of the pulsed periods and apply the harvested and stored energy to power rails of the power converter during the leading edges of the pulsed periods.
 24. The apparatus of claim 16, wherein the power converter is further configured to control energy flow to or from the electric machine using one of the following protocols: (a) Pulse Width Modulation (PWM); (b) Direct Torque Control (DTC), (c) hysteresis, or (d) current modulation.
 25. The electric machine of claim 16, wherein the electric machine is one of the following: a DC motor or generator; an AC motor or generator; an induction machine; a pulse width controlled electric machine; or a synchronous AC machine. 